Near-infrared Reflectance Spectra of Mixtures of Kaolin-group Minerals: Use in Clay Mineral Studies

-Near-infrared (NIR) reflectance spectra for mixtures of ordered kaolinite and ordered dickite have been found to simulate the spectral response of disordered kaolinite. The amount of octahedral vacancy disorder in nine disordered kaolinite samples was estimated by comparing the sample spectra to the spectra of reference mixtures. The resulting estimates are consistent with previously published estimates of vacancy disorder for similar kaolin minerals that were modeled from calculated X-ray diffraction patterns. The ordered kaolinite and dickite samples used in the reference mixtures were carefully selected to avoid undesirable particle size effects that could bias the spectral results. NIR spectra were also recorded for laboratory mixtures of ordered kaolinite and halloysite to assess whether the spectra could be potentially useful for determining mineral proportions in natural physical mixtures of these two clays. Although the kaolinite-halloysite proportions could only be roughly estimated from the mixture spectra, the halloysite component was evident even when halloysite was present in only minor amounts. A similar approach using NIR spectra for laboratory mixtures may have applications in other studies of natural clay mixtures. Key Words--Dickite, Interstratification, Kaolinite, Near-infrared spectroscopy, Order-disorder, X-ray powder diffraction. I N T R O D U C T I O N X-ray powder diffraction (XRD) modeling techniques have been applied to a number of important clay mineral problems, including studies of structural disorder (Plan~on and Tchoubar, 1977) and the determinat ion of layer proportions in interstratified clay minerals (Reynolds, 1980). Some clay mineral characteristics, however, are inherently difficult to analyze by XRD methods, either because the diffraction features of interest are ambiguous, or because short-range structural phenomena are involved, which do not result in coherent X-ray scattering behavior. In this study, near-infrared reflectance spectra are used to address two such problems involving kaolin-group minerals. In each experiment spectral measurements were recorded for mineral mixtures that simulated natural clay materials. The first experiment dealt with the estimation of octahedral vacancy disorder in kaolinite, a question that has previously been addressed by XRD modeling techniques, but which has not been corroborated by other analytical methods (Plan~on and Tchoubar, 1977). The second experiment involved the quantitative analysis of intimate mixtures of ordered kaolinite and halloysite, a problem that has not been resolved by XRD or other methods (Brindley et al., 1963).